ABSTRACT Prolonging the duration of the partial clinical remission (PCR), or `honeymoon' phase, of type 1 diabetes (T1D) improves glycemic control and reduces long-term complications. Recent studies suggest the exciting possibility that vitamin D supplementation, a safe and easy-to-implement therapy in children, may lengthen PCR and increase residual beta cell function (RBCF). However, existing studies employed a suboptimal vitamin D dose or lacked a standardized insulin treatment protocol, precluding solid conclusions and preventing the field from moving forward with translation to clinical practice. This trial's rationale is to securely establish the effect of an adequate dose of vitamin D on PCR and RBCF. We hypothesize that vitamin D will increase RBCF and prolong PCR. The primary aim is to determine the effect of adjunctive vitamin D on RBCF and PCR in youth with T1D maintained on a standardized insulin protocol. We propose a 12-month randomized, double-blind, placebo- controlled, parallel design trial of ergocalciferol vs. placebo in 40 subjects of 10-21 years with newly-diagnosed T1D. The primary outcome is the change over time in stimulated C-peptide (a measure of RBCF). Secondary outcomes include change over time in insulin-dose- adjusted-hemoglobin-A1c (HbA1c) (IDAA1C; a measure of PCR), HbA1c, and total daily dose of insulin. Mechanistic studies will explore whether beneficial effects of vitamin D are associated with increased GLP-1 levels or decreased inflammatory markers, and whether response to vitamin D is impacted by T1D-risk polymorphisms. If our hypotheses are true, these findings may completely alter the approach to the early management of T1D, with strong emphasis on prolonging the honeymoon phase using a readily available and easily affordable vitamin D while maintaining these patients on a standardized insulin treatment regimen.